tests/qemu-iotests: print intent to run a test in TAP mode

When running I/O tests using TAP output mode, we get a single TAP test
with a sub-test reported for each I/O test that is run. The output looks
something like this:

 1..123
 ok qcow2 011
 ok qcow2 012
 ok qcow2 013
 ok qcow2 217
 ...

If everything runs or fails normally this is fine, but periodically we
have been seeing the test harness abort early before all 123 tests have
been run, just leaving a fairly useless message like

  TAP parsing error: Too few tests run (expected 123, got 107)

we have no idea which tests were running at the time the test harness
abruptly exited. This change causes us to print a message about our
intent to run each test, so we have a record of what is active at the
time the harness exits abnormally.

 1..123
 # running qcow2 011
 ok qcow2 011
 # running qcow2 012
 ok qcow2 012
 # running qcow2 013
 ok qcow2 013
 # running qcow2 217
 ok qcow2 217
 ...

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220509124134.867431-2-berrange@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/testrunner.py

@@ -361,6 +361,9 @@ class TestRunner(ContextManager['TestRunner']):
                                      starttime=start,
                                      lasttime=last_el,
                                      end = '\n' if mp else '\r')
+        else:
+            testname = os.path.basename(test)
+            print(f'# running {self.env.imgfmt} {testname}')
 
         res = self.do_run_test(test, mp)